Strange font-style / font-weight on CPT list columns screen

This is happening because of this CSS rule that is generated in load-styles.php:

.widefat thead th:last-of-type {
    -moz-border-radius-topright: 3px;
    -khtml-border-top-right-radius: 3px;
    -webkit-border-top-right-radius: 3px;
    border-top-right-radius: 3px;
}

I’m not sure why this is here. To remove it, you should make a custom admin theme plugin to overwrite that CSS rule.

First create a folder in your /wp-content/plugins/ directory named wpse_34959 (or whatever plugin name you want). In that folder, create wpse_34959.php with the following contents:

<?php
/*
Plugin Name: My Admin Theme
Plugin URI: http://yoursite.com
Description: My WordPress Admin Theme - Upload and Activate.
Author: Your Name
Version: 1.0
*/

function my_admin_head() {
        echo '<link rel="stylesheet" type="text/css" href="' .plugins_url('wp-admin.css', __FILE__). '">';
}

add_action('admin_head', 'my_admin_head');

?>

In that same plugin directory, create a file named wp-admin.css with the following contents:

.widefat thead th:last-of-type {
    -moz-border-radius-topright: 0px;
    -khtml-border-top-right-radius: 0px;
    -webkit-border-top-right-radius: 0px;
    border-top-right-radius: 0px;
}

Activate the plugin via the WordPress Admin interface, and those styles that is causing that behavior should now be overridden.

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